Experiments with a Skywatcher Auto Focuser

[Gweedo]

I’ve been thinking about upgrading the focuser of my 250mm Dob for a while now and I was leaning towards a micro focuser upgrade when I saw the site reviews of the Skywatcher Autofocuser and thought that would possibly be a better (and cheaper!) option. Before I could think much more about it, I placed my order with FLO, and as usual, it arrived first thing next morning. Excellent service yet again from the guys at FLO.

In the excellent review that Tophouse wrote (http://stargazerslounge.com/member-equipment-reviews/97578-skywatcher-auto-focuser.html ) he mentioned 2 aspects that he thought could be improved:

Firstly, the speed control was a rotary potentiometer, making it a 2 handed operation and secondly, that there was no way of de-clutching the drive from the focus – in effect, you were stuck with motor drive whether you liked it or not. There was some discussion on the thread about whether or not you could damage the motor drive by forcing the manual focus. Obviously, this would not be a good thing.

Having installed the drive on the scope, I thought about what could be done to remedy the drive issue, and how I’d go about making a modification. In essence, the ‘design brief ‘ I gave myself was:

1. It had to be able to de-clutch the drive to allow manual focus.

2. It had to be simple.

3. Preferably, it had to be easy to make, and retain or not damage the existing fittings.

Now, I’m fortunate to have my own small lathe and milling machine which puts me at a great advantage. I thought of all sorts of spring operated clutch mechanisms, but I thought that the simplest option would be to re-site the motor drive and use a pulley system to operate the focus. Also, by using a small to large pulley, I could make the fine-tuning of the focus even finer. So, on Saturday afternoon, I set to work.

The hand focus wheel is a 2 part assembly – an alloy casting and what appears to be a steel fitting (It turns out, it is chromed brass). They are loc-tited together, so it requires a bit of force to unscrew one from the other. What you get is a brass fitting with a 6mm male thread and the alloy wheel.

With a bit of aluminium in the lathe, I turned a replacement for the chromed fitting, incorporating a pulley wheel and a thread so that the alloy wheel can be fitted on this new piece. I tapped a 3mm hole for the retaining grub screw. A final polish and the part was finished.

Before stopping for the night, I decided to remove the coupling fitting on the motor drive ready for the next stage. It wasn’t difficult, but you should look at the postscript….

This morning, I set to work making a new mounting plate out of aluminium. A pretty straightforward job involving some measuring, drilling, cutting and filing to shape. It was cleaned up, de-greased, primer coated and painted.

I turned another pulley; this time for the drive. The drive shaft is 6mm with a flattened section, so this limits the size of the pulley you can make. As I said earlier, I decided to make the drive pulley smaller in order to alter the ratio. Of course, you could opt for a 1:1 ratio if you wanted to.

All done, it was a matter of assembling it all. The picture below shows it all assembled. I thought of using a wide rubber band as the drive belt between the drive and the focus, the thought being that this would have sufficient friction to drive the focus, but also allow you to use manual drive when you wanted to.

It turns out it all works, but here’s a summary:

Pros:

You can use manual drive to override motor drive if you’d like to.

I think that the drive motor location is neater.

Cons:

I know that I wasn’t using the drive band that I wanted to, but I have my doubts that it would offer sufficient friction (especially if I had a camera mounted) to move the focus tube. It would be relatively easy to ramp up the friction, but that sort of defeats the object of the exercise really.

The loading is not in line with the drive shaft, which puts wear on one side of the drive and the focus – not really good engineering practice.

The rubber band/drive is open and not neat.

So, in the final analysis, it works, but it’s not great – back to the drawing board for me……..

The postscript.

Even though I found this out halfway through the project, I carried on regardless, just to see it through really.

You’ll recall that there was some discussion in the original review thread about manually moving the focus against the motor drive and concern that this could damage the motor. This is the drive coupling disassembled:

As you can see, there is a retaining cap, in which sits an O ring and into this assembly screws the connector to the focus drive. The mount is not directly attached to the motor: what happens is that the O ring forms a friction grip onto the motor drive shaft. The friction is increased by screwing in the shaft connector (which is gripped by the grub screw on the wider section). To reduce the mount/drive friction all you need to do is loosen the grub screw on the wider section, unscrew the connector a little, tighten up the grub screw again, and that should offer less of a friction grip between motor drive and connector. In short, you are not in much danger of damaging the motor by manually moving the focus as it should slip over the motor drive shaft. I’d guess that it would loosen over time if you did it a lot, and seeing that the way they are fitted from the factory, it looks more as though this is a final protection measure for the motor, rather than as a way to manually over-ride the drive.

[Aoraki]

I do like to see things like this, lovely bit of engineering.